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Mobile Ad Hoc Networks

Parth Panchal

1

, Meghana Shroff

2

Student, III year, Bachelor of Technology (Electronics)

Mukesh Patel Institute of Technology Management and Engineering NMIMS, SVKM Mumbai

1panchal.parth@outlook.com

2meghanashroff@gmail.com

Abstract— A mobile ad hoc network (MANET) is a

self-configuring infrastructure-less network of mobile devices connected by wireless. The primary challenge in building a MANET is equipping each device to continuously maintain the information required to properly route traffic. AODV is a novel algorithm for the operation of such ad hoc networks. This routing algorithm is quite suitable for a dynamic self-starting network, as required by users wishing to utilize ad hoc networks. AODV provides loop free routes even while repairing broken links. We attempt to build such a network using AODV.

Keywords

MANET, AODV, Ad hoc, network simulator NS

I. INTRODUCTION

Laptop computers continue to show improvements in convenience mobility memory capacity and availability of disk storage. The smaller computers can be equipped with gigabytes of disk storage high resolution color displays pointing devices and wireless communications adapters. Moreover because many of these small (in size only) computers operate with battery power users are free to move about at their convenience without being constrained by wires. More recently the interest in this subject has grown due to availability of license free wireless communication devices that users of laptop computers can use to communicate with each other. Several recent papers on this topic have focused on the algorithmic complexity of choosing the optimal set of ad hoc routers while others have proposed new routing solutions leveraging features from the existing Internet routing algorithms.

As mentioned earlier, each device in a MANET is free to move independently in any direction, and will therefore change its links to other devices frequently. Each must forward traffic unrelated to its own use, and therefore be a router. Such networks may operate by themselves or may be connected to the larger Internet. MANETs are a kind of Wireless ad hoc network that usually has a routable networking environment on top of a Link Layer ad hoc network.

A. Destination-Sequenced Distance-Vector

The Destination-Sequenced Distance-Vector (DSDV) Routing Algorithm is based on the idea of the classical

Bellman-Ford Routing Algorithm with certain

improvements. Every mobile station maintains a routing table that lists all available destinations, the number of hops to reach the destination and the sequence number assigned by the destination node. The sequence number is

used to distinguish stale routes from new ones and thus avoid the formation of loops. The stations periodically transmit their routing tables to their immediate neighbors. A station also transmits its routing table if a significant change has occurred in its table from the last update sent. So, the update is both time-driven and event-driven.

The routing table updates can be sent in two ways: a “full dump” or an incremental update. A full dump sends the full routing table to the neighbors and could span many packets whereas in an incremental update only those entries from the routing table are sent that has a metric change since the last update and it must fit in a packet. If there is space in the incremental update packet then those entries may be included whose sequence number has changed. When the network is relatively stable, incremental updates are sent to avoid extra traffic and full dump are relatively infrequent. In a fast-changing network, incremental packets can grow big so full dumps will be more frequent.

B. Dynamic State Routing (DSR)

The key distinguishing feature of DSR is the use of link state routing. The sender/source knows the complete hop-by-hop route. The routes are stored in route cache. A route discovery process is used to dynamically discover routes. Route discovery works by flooding the network with RREQ packets. Each node receiving an RREQ rebroadcasts it, unless it is the destination or it has a route to the destination in its route cache. Such a node replies to the RREQ with a route reply (RREP) packet that is routed back to the original source. RREQ and RREP packets are also source routed. The RREQ builds up the path traversed across the network. The RREP routes itself back to the source by traversing this path backward. The route carried back by the RREP packet is cached at the source for future use. If any link on a source route is broken, the source node is notified using a route error (RERR) packet. The source removes any route using this link from its cache. A new route discovery process must be initiated by the source if this route is still needed. DSR makes very aggressive use of source routing and route caching.

C. Ad hoc On-Demand Distance Vector (AODV)

AODV discovers routes on an as needed basis via a similar route discovery process. However, AODV adopts a very different mechanism to maintain routing information. It uses traditional routing tables, one entry per destination. It builds routes between nodes only when it is required by

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source nodes and also it maintains these routes as long as they are needed by these sources. AODV forms trees which connect multicast group members. The trees are composed of the group members and the nodes needed to connect the members. AODV uses sequence numbers to ensure the freshness of routes. It is loop-free, self-starting, and scales to large numbers of mobile nodes.

Although AODV does not depend specifically on particular aspects of the physical medium across which packets are disseminated, its development has been largely motivated by limited range broadcast media, such as those utilized by infrared or radio frequency wireless communications adapters. We do not make any attempt to use specific characteristics of the physical medium in our algorithm nor to handle the problems posed by channelization needs of radio frequency transmitters. Nodes that need to operate over multiple channels are presumed to be able to do so. The algorithm works on wired media as well as wireless media as long as links along which packets may be transmitted are available. The only requirement placed on the broadcast medium is that neighboring nodes can detect each other’s broadcasts. AODV uses symmetric links between neighboring nodes. It does not attempt to follow paths between nodes when one of the nodes cannot hear the other one however we may include the use of such links in future enhancements.

II. AODVPROTOCOL

AODV builds routes using a route request (RREQ) and route reply (RREP) message cycle. When a source node desires a route to a destination for which it does not already have a route, it broadcasts a RREQ packet to the network. Nodes receiving this packet update their information for the source node and set up backward pointers to the source node in the routing tables. In addition to the source node's IP address, current sequence number, and broadcast ID, the RREQ also contains the most recent sequence number for the destination of which the source node is aware.

Fig 1: Route discovery

Fig 2: RREQ Packet Format

A node receiving the RREQ may send a RREP if it is either the destination or if it has a route to the destination

with corresponding sequence number greater than or equal to that contained in the RREQ. If this is the case, it unicasts a RREP back to the source. Otherwise, it rebroadcasts the RREQ. Nodes keep track of the RREQ's source IP address and broadcast ID. If they receive a RREQ which they have already processed, they discard the RREQ and do not forward it.

Fig 3: Route reply

Figure 4: RREP Packet Format

As the RREP propagates back to the source, nodes set up forward pointers to the destination. Once the source node receives the RREP, it may begin to forward data packets to the destination. If the source later receives a RREP containing a greater sequence number or contains the same sequence number with a smaller hop count, it may update its routing information for that destination and begin using the better route.

Fig 5: AODV Algorithm

As long as the route remains active, it will continue to be maintained. Once the source stops sending data packets, the links will time out and eventually be deleted from the intermediate node routing tables. If a link break occurs while the route is active, the node upstream of the break propagates a route error (RERR) message to the source node to inform it. After receiving the RERR, if the source node still desires the route, it can restart route discovery.

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The algorithm’s primary objectives are:

• To broadcast discovery packets only when necessary • To distinguish between local connectivity management

neighborhood detection and general topology maintenance

• To disseminate information about changes in local connectivity to those neighboring mobile nodes that are likely to need the information

Multicast routes are set up in a similar manner. A node wishing to join a multicast group broadcasts a RREQ with the destination IP address set to that of the multicast group and with the 'J'(join) flag set to indicate that it would like to join the group. Any node receiving this RREQ that is a member of the multicast tree that has a fresh enough sequence number for the multicast group may send a RREP. As the RREPs propagate back to the source, the nodes forwarding the message set up pointers in their multicast route tables. As the source node receives the RREPs, it keeps track of the route with the freshest sequence number, and beyond that the smallest hop count to the next multicast group member. After the specified period, the source node will unicast a Multicast Activation (MACT) message to its selected next hop. This message serves the purpose of activating the route. A node that does not receive this message that had set up a multicast route pointer will timeout and delete the pointer. If the node receiving the MACT was not already a part of the multicast tree, it will also have been keeping track of the best route from the RREPs it received. Hence it must also unicast a MACT to its next hop, and so on until a node that was previously a member of the multicast tree is reached.

AODV maintains routes for as long as the route is active. This includes maintaining a multicast tree for the life of the multicast group. Because the network nodes are mobile, it is likely that many link breakages along a route will occur during the lifetime of that route. The only other circumstance in which a node may change the destination sequence number in one of its route table entries is in response to a lost or expired link to the next hop towards that destination.

Once the next hop becomes unreachable, the node upstream of the break propagates an unsolicited RREP with a fresh sequence number i.e. a sequence number that is one greater than the previously known sequence number, and hop count of ∞ to all active upstream neighbors. Those nodes subsequently relay that message to their active neighbors and so on. This process continues until all active source nodes are notified. It terminates because AODV maintains only loop free routes and there are only a finite number of nodes in the ad hoc network. Consider node A wants to connect to node B, but the link is broken in between. This means that node A knows that the link to node B is down and increments the Sequence number and broadcast it.

III. SIMULATION IN NS2

We have simulated AODV using an event simulator NS2. The main objective of our simulations is to show that on demand route establishment with AODV is both quick and accurate. Additional objectives include showing that AODV scales well to large networks and determining the optimal value for each of the necessary parameters.

A. Simulation environment

The simulation experiment is carried out in LINUX (Ubuntu 12.04). The detailed simulation model is based on network simulator-2 (ver-2.36), is used in the evaluation. The NS instructions can be used to define the topology structure of the network and the motion mode of the nodes, to configure the service source and the receiver, to create the statistical data track file and so on.

The following simulation results were obtained

Figure 6: Simulation scenario in the beginning

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Figure 8: Simulation scenario 2

Figure 9: Simulation scenario 3

Figure 10: Simulation scenario 4

B. Performance Analysis

For AODV, packet delivery ratio is independent of offered traffic load, delivering between 85% and 100% of the packets in all cases. The lazy approach used by AODV to build the routing information as and when it is created makes it more adaptive and results in better performance (high packet delivery fraction and lower average end-to-end packet delays). In the presence of high mobility, link failures can happen very frequently. Link failures trigger new route discoveries in AODV since it has at most one route per destination in its routing table. Thus, the frequency of route discoveries in AODV is directly proportional to the number of route breaks.

Figure 11: Simulation scenario 5

Figure 12: Simulation scenario 6

Performance of AODV protocols in MANET can be realized by quantitative study of values of different metrics used to measure performance of routing protocols which are as follows:

1) Average End-to-End Delay: It is defined as average time taken by data packets to propagate from source to destination across a MANET. This includes all possible delays caused by buffering during routing discovery latency, queuing at the interface queue, and retransmission delays at the MAC, propagation and transfer times. The lower value of end to end delay means better performance of the protocol.

End to end delay = Σ (arrive time - send time)

2) Packet Delivery Ratio: It is a ratio of the number of packets received by the destination to the number of packets sent by the source. This illustrates the level of delivered data to the destination. The greater value of packet delivery ratio means better performance of the protocol.

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3) Packet Loss: It is the measure of number of packets dropped by nodes due to various reasons. The lower value of the packet lost, the better performance of the protocol.

Packet lost = No of packet sent – No of packets received. III. CONCLUSION

In summary, we present a distance vector algorithm that is suitable for use with ad hoc networks. AODV has the following features:

• Nodes store only the routes that are needed • Need for broadcast is minimized

• Reduces memory requirements and needless duplications • Quick response to link breakage in active routes

• Loop free routes maintained by use of destination • Sequence numbers scalable to large populations of nodes

Compared to DSDV and other algorithms which store continuously updated routes to all destinations in the ad hoc network our algorithm has longer latency for route establishment. But within the limits imposed by worst case route establishment latency, AODV is a sensible choice for ad hoc network establishment. We look forward to further development of the protocol for quality of service intermediate route rebuilding and various interconnection topologies with fixed networks and the Internet.

A. Future Scope

We believe strongly in the scope of MANET, essentially in the form of Vehicular Ad hoc Network. Vehicular ad hoc network is a special form of MANET which is a vehicle to vehicle & vehicle to roadside wireless communication network. It has great scope in vehicle collision warning, security distance warning, driver

assistance, cooperative driving, and cooperative cruise control, dissemination of road information, internet access, map location, automatic parking and driverless vehicles. It also has applications for emergency services, conferencing battlefield communications and community based networking.

ACKNOWLEDGMENT

We would like to express our sincere gratitude towards Mr. Sharad Wagh, Assisstant Professor, NMIMS, for his technical guidance and constant encouragement throughout the project, and for his priceless inputs and patience to deal with our queries.

We would also like to thank the authority of SVKM’s NMIMS for providing us with a good environment and facilities to complete this project. Without help of the aforementioned, we would have faced many difficulties while doing this project.

REFERENCES

[1] C. Perkins, E. Belding-Royer, and S. Das. Ad hoc On- Demand

Distance Vector (AODV) Routing. RFC 3561 (Experimental), July

2003.

[2] Y. C. Hu, D. Johnson, and A. Perrig. -SEAD: Secure efficient distance vector routing for mobile wireless ad hoc networks,‖ in

Fourth IEEE Workshop on Mobile Computing Systems and Applications (WMCSA '02), 2002, p. 313.

[3] Ashokan, Manel Guerrero Zapata. -Securing ad hoc routing protocols, 2002,‖ in ACM workshop on Wireless security [4] S. Kent, C. Lynn, J. Mikkelson, and K. Seo. Secure border gateway

protocol (S-BGP) -real world performance and deployment issues,

2000.

[5] A. Nagaraju, Dr S Ramachandram and Dr C. R. Rao. Applying heuristic technique to ad-hoc on demand distance vector routing to reduce broadcast, ‖ in the World Congress on Engineering Vol II, WCE 2007, London, U.K, 2007

[6] NS-2, The ns Manual (formally known as NS Documentation) available at http: //www. isi.edu/nsnam/ ns/doc.

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